At the risk of sounding disrespectful, her potty issues are not her fault. You neglected to train her and now you want to put her down for not being potty trained. The main reasons why pets end up in shelters is because of the owner! The owner does not want to take the time to properly trained and socialized their pets.
If she isn't having severe medical issues that has diminished her quality of life or is causing her pain, why would you want to put down? I bet if you took the time to properly trained her, all those things you want to do with your dog you can do. If you want her to sleep in bed with you, potty trained her. It really is that simple. This isn't you dog's fault and she shouldn't be put to death because she was not potty trained.
If you are just plain tired of her, then rehome her. However that would be a little cold to rehome a senior dog that has only known one home her entire life because her owner didn't potty trained her and is now tired of cleaning up her crap.......that really sucks.